Waste products are produced during the refining of petroleum, for example, heavy oily sludges, biological sludges from waste water treatment plants, activated sludges, gravity separator bottoms, storage tank bottoms, oil emulsion solids including slop oil emulsion solids or dissolved air flotation (DAF) float from floculation separation processes. Waste products such as these may create significant environmental problems because they are usually extremely difficult to convert into more valuable, useful or innocuous products. In general, they are usually not readily susceptible to emulsion breaking techniques and incineration which requires the removal of the substantial amounts of water typically present in these sludges would require elaborate and expensive equipment. For this reason, they have often been disposed of in the past by the technique known as "land farming" by which the sludge is worked into the land to permit degradation by bacterial action. Resort to these methods has, however, become more limited in recent years with increasingly stringent environmental controls and increases in the amount of such waste products produced in refineries. In particular, the use of land farming is likely to encounter more stringent regulation in the future because of the potential for pollution, both of ground water and the air.
A process for disposing of petroleum refinery sludges and other wastes is disclosed in U.S. Pat. No. 3,917,564 (Meyers) and this process has been shown to be extremely useful. In it, sludges or other by-products of industrial and other community activity are added to a delayed coker as an aqueous quench medium during the quench portion of the delayed coking cycle. The combustible solid portions of the byproduct become a part of the coke and the non-combustible solids are distributed throughout the mass of the coke so that the increase in the ash content of the coke is within commercial specifications, especially for fuel grade coke products. As shown in U.S. Pat. No. 3,917,564, sludges which may be treated by this method include petroleum refinery slop emulsions, biological sludges and sludges containing large amounts of used catalytic cracking catalyst mixed with biological wastes.
Another proposal for dealing with petroleum sludges is disclosed in U.S. Pat. No. 4,666,585 (Figgins) which discloses a process in which petroleum sludges are recycled by adding them to the feedstock to a delayed coker before the quenching cycle so that the sludge, together with the feed, is subjected to delayed coking. This process has the desirable aspect of subjecting the combustible portion of the sludge to the high coking temperatures so that conversion either to coke or to cracked hydrocarbon products, takes place. However, the presence of water in the sludge tends to lower the coking temperature unless compensation is made for this factor, for example, by increasing the operating temperature of the furnace and this may decrease the yield of the more desirable liquid products from the delayed coking process.
In addition, the amount of sludge which may be added to the coker feed is limited by the presence of the relatively large amounts of water in the sludge. As described in the patent, the amount of sludge is limited to 0.01 to 2 weight percent.
As described in U.S. Pat. No. 4,874,505, the waste recycling operation may be improved by segregating refinery sludges and separately injecting them into the delayed coker at different times during the delayed coking cycle: the oily sludges such as slop oils, storage tank sludges and gravity separator skimmings are injected into the coker drum during the coking cycle and the more watery sludges such as DAF float or biosludge are injected during the quench cycle. Reference is made to U.S. Pat. No. 4,874,505 for a full description of the process.